For several decades scientists have been using telescopes to scan the heavens for unnatural-looking radio or optical transmissions coming from intelligent alien life. With this search for extraterrestrial intelligence (SETI) having so far failed to pick up a single signal, however, researchers in the US now believe it is worth extending the search beyond electromagnetic waves and start paying attention to neutrinos.

John Learned of the University of Hawaii and colleagues have worked out that advanced alien civilizations could send messages within the Milky Way using neutrinos, and that these messages could be picked up using neutrino detectors currently under construction here on Earth (arXiv:0805.2429).

This may seem like an odd proposal because neutrinos are in fact extremely difficult to detect, since they interact very weakly with ordinary matter. This means that neutrino observatories are hard to build — requiring vast amounts of detecting material and located deep underground or under sea or ice — and even the most sophisticated detect very few particles.

Low-noise communications

But Learned and colleagues Sandip Pakvasa of the University of Hawaii and Tony Zee at the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara believe that neutrino communications offer several advantages over electromagnetic waves. Radio or optical signals can be blocked by material within the galaxy, for example, and the radiation that does make it through is obscured by numerous sources of electromagnetic noise. Neutrinos, on the other hand, pass through the galaxy virtually unimpeded and, if highly energetic, are extremely rare and therefore do not suffer from background interference.

The US researchers assume that alien neutrino beams would be pulsed and directional, and that the messages would probably be sent in something akin to Morse code – with a varying time interval between pulses used to encode the information. They also believe that an advanced civilization would not use neutrinos with energies of less than about a million electron-volts, in order to avoid any interference from neutrinos produced by natural radioactive decay and solar processes. They suggest that SETI hunters should target a specific energy of 6.3 petaelectron-volts (PeV) , which is 6.3x1015 eV. This is the energy at which the “Glashow resonance” takes place, whereby an electron antineutrino interacts with an electron to create a W- particle.

Enormous amounts of energy

Learned and colleagues have put forward two ways of producing such neutrinos. The first of these involves colliding electrons and positrons at an energy equal to the mass of the Z0 particle, a relatively simple process in principle but one that would require enormous amounts of energy – about 3% of the Sun’s power output for neutrinos to be sent over a distance of 3000 light years.

The second approach instead involves firing protons at a target, accelerating the pions that emerge to around 30 PeV, and then separating out the pion decay products (muons and muon neutrinos). This process could in fact be carried out using the power output of proposed thermonuclear power plants, and would have the added advantage of being able to produce both neutrinos and antineutrinos (switching between the two would provide an additional way of encoding messages). Accelerating the pions to such high energies would be a huge challenge but “not wildly implausible for a future civilization”, according to Learned.

Next-generation neutrino telescopes

As to our ability to intercept such messages, the researchers believe that this will be possible soon using next-generation neutrino telescopes with a detector volume of around 1 km3. These include the IceCube telescope under construction at the South Pole and a possible successor to the ANTARES, NEMO and NESTOR observatories in the Mediterranean. This is a view shared by Francis Halzen, principal investigator of IceCube. Indeed, observations would be clear cut since there are no known natural mechanisms for making neutrinos at 6.3 PeV — detecting two or more of these particles would be a tell-tale sign that they had been artificially produced.

Learned and colleagues believe it is important to keep neutrino telescopes running for extended periods. They point out that extraterrestrial civilizations would have no way of knowing when to transmit, since their messages may take tens of thousand of years to reach their intended recipients (the Milky Way is thought to be some 100,000 light years across) and it would be impossible to predict exactly when a life-friendly planet would become industrialized. Any intelligent beings out there may therefore decide to send messages periodically, and we cannot predict what this period would be, Learned adds. “If there are signals there it will be obvious,” he says. “But we will have to keep looking.”